1. Field of the Invention
The present invention generally relates to a living tissue stimulator system and, more particularly, to a living tissue stimulation system with a rechargeable power source and charge control means.
2. Description of the Prior Art
In recent years advances have been made and systems proposed for the use of electrical stimulation of living tissue, such as nerves, muscles and the like. U.S. Pat. No. 3,083,712 describes a device for producing electrical muscle therapy. The use of electrical stimulation of living tissue in the treatment of epilepsy, cerebral palsy and spasticity is described in an article entitled "Effect of Chronic Stimulation of Anterior Cerebellum on Neurological Disease", Lancet 1:206, 1973. The stimulation of different living tissues is described in other prior art publications, e.g., a report entitled "Functional Neuromuscular Stimulation", published by the National Academy of Sciences, Washington, D.C. in 1972.
The most widely used and best known device which provides electrical stimulation of living tissue is the cardiac pacer. Briefly, the cardiac pacer is a device designed to provide electrical stimulating pulses to the heart. One type of cardiac pacer, known as the fixed rate cardiac pacer, provides stimulating pulses to the heart, irrespective of proper heart beating, while another type, known as the demand inhibited cardiac pacer, provides a stimulating pulse only when the heart fails to provide a proper natural pulse. The pacer, regardless of type, usually includes a source of power, e.g., a battery which powers a pulse generator. The latter generates the pulses which are applied to the patient's heart by means of one or more electrodes located at the heart and electrically connected to the generator by electrode leads.
The very early implanted cardiac pacers, i.e., those implanted inside the patient's body, included a small battery which required periodic replacement, thus necessitating periodic surgeries. To overcome this disadvantage a cardiac pacer was developed which includes a battery which is rechargeable with energy from a source, external to the body. Such a pacer is described in prior-art references, including U.S. Pat. Nos. 3,867,950, 3,888,260 and 3,942,535.
Briefly, the implanted pacer includes a charging circuit which is energized by external energy, such as an alternating magnetic field. The charging circuit in turn produces a charging current, which is made to flow to the rechargeable battery, thereby recharging the latter. The implanted charging circuit and the rechargeable battery can of course be used in any implanted living tissue stimulator or pacer, used to stimulate tissue, other than the heart. As used herein the term pacer intends to refer to any pacer implanted in a patient's body to provide stimulating pulses to the heart or any other tissue to be stimulated. That is, it is not intended to be limited to a cardiac pacer.
Although the implanted charging circuit with the rechargeable battery eliminate the need for frequent surgeries to replace batteries, their use in the pacer has developed many design problems, only some of which have been solved by the prior art. It was discovered that when recharging the battery it is important to be able to monitor the charging current as well as the battery state of charge, to insure that the battery is being properly charged and that the charging current does not exceed a maximum current level, which may permanently damage the battery. Also, it is important to prevent the battery from being overcharged above a safe level.
In U.S. Pat. No. 3,888,250 an implanted cardiac pacer is described in which the charging current is monitored and a signal, whose frequency is related to the current amplitude, is transmitted external to the patient. In U.S. Pat. No. 3,942,535 similar means are described. More recently, a tissue stimulation system has been proposed in which at least one switch is incorporated in the path of the charging current and special sensors are coupled to the battery to determine its state of charge. The combined function of the switch and the special sensors is to control the battery charging so that it does not exceed a desired charged state.
These battery-protection devices, namely, the switch and the special sensors perform their intended function only when they perform properly. However, if one or more of them fail, a danger exists that they will either not protect the battery from becoming over-charged above a safe level and thereby become permanently damaged, or that they will cause signals to be transmitted external to the patient which may be interpreted as indicating that the battery is properly charged, when in fact it is not. In the prior art system the battery-protection devices, are assumed to be fail-proof, which in fact they are not. The prior art system does not include any means, whether implanted or external, with which the malfunctioning of one or more of these battery-protection devices can be determined, nor means for providing a safe charging current to the battery even when one or more of the battery-protection devices have failed.
The life of a patient with an implanted pacer often depends on the pacer's proper operation, for which a properly charged undamaged battery is an absolute requirement. A presently existing implanted pacer with a rechargeable battery has an expected life of many years, e.g., 10 years or more, if only the battery can be protected properly by the protection devices, to prevent the battery from being overcharged above a safe level and/or preventing the battery from being charged by an excessively high charging current. Thus, a need exists for a stimulation system incorporating, an implanted pacer with a rechargeable battery, in which the battery, though protected by protection devices, can be charged safely even when one or more of these protection devices fail. Alternately stated, a need exists for a living tissue stimulation system, with an implanted pacer with a rechargeable battery and battery-protection devices, the system being operable so as to sense failure of any of the devices and in spite of such failure or failures be able to safely charge and protect the battery from permanent damage.